Even though Donald and the others are well-defined animals, it doesn't prevent them from disguising themselves as non-anthropomorphic animals a couple of times. For people who are into fursuiting, this is quite nice, especially adding to the charm is the recurring situation of getting stucked and humiliated in that animal costume... :

Donald is forced by Scrooge to dress up in a full-body dog outfit in order to impersonate a rich lady's pet dog. He has to eat dog food, lick her... and in the end, he gets stuck in the outfit and Scrooge rewards him with his efforts... with a bone.

Donald is getting jealous of Daisy dating a much more cooler and handsome duck who works at a movie set. He decides to dress up in a full-body monkey outfit and commit sabotage on his job. It fails, his outfit gets stucked and he ends up in a cage at a zoo, angrily looking at Daisy and her new beau as they walk by.

Donald puts on a full-body bear outfit, gets strucked by a tranquilizer dart and becomes a star at the circus.

Donald wants to get a dog. A bank robber who is running away from the police finds by sheer accident a full-body dog outfit, wears it and manages to escape the cops. But Donald finds him and decides to take him as a pet. The robber has to play along, almost gets drowned (because he can't swim, which surprises Donald who thinks all dogs can swim, starting from their birth) and he gets sick of the act, but gets captured by the dog pound. He protests by speaking, saying he is not a dog and is wearing a disguise, but the dog-catcher replies "this is what they all say!". Even Donald is surprised by his reply.